Magnetic field generating device for MRI

ABSTRACT

It is an object of the present invention to provide a magnetic field generator for MRI with which it is possible to lower the residual magnetism and eddy current within pole pieces generated by the effect of the pulse current flowing through Gradient magnetic field coils, without decreasing the field uniformity within the air gap. Pole pieces  40  in which a main component  41  consisting of laminated silicon steel sheets is effectively combined with a magnetic annular protrusion  42  disposed on the side on the main component  41  facing the air gap, the result of which is the formation of a static magnetic field having the desired uniformity within the air gap without leading to a state of magnetic saturation in the vicinity of the magnetic annular protrusion  42 . This also makes possible a reduction in the residual magnetism and eddy current within the pole pieces  40  generated by the effect of the pulse current flowing through Gradient magnetic field coils.

RELATED CASES

This application is related to PCT/JP/99/01944 filed Oct. 21, 1999.

TECHNICAL FIELD

The present invention relates to a magnetic field generator used in amedical-use magnetic resonance imaging (hereinafter referred to as MRI)device, and more particularly to an MRI magnetic field generator withreduced residual magnetism and eddy current generated by the effect ofthe pulse current flowing through Gradient magnetic field coils.

BACKGROUND ART

FIGS. 13(a) and (b) illustrate a known structure of an MRI magneticfield generator. In this structure, each of a pair of pole pieces 2 isfastened, with the pole pieces 2 facing each other, at one end of eachof a pair of permanent magnet structures 1 comprising a plurality ofblock-shaped R-Fe-B-based magnets that have been integrated as the fieldgeneration source the other ends of the permanent magnet structures areconnected to a yoke 3, and a static magnetic field is generated withinthe air gap 4 between the pole pieces 2.

In the figure, 5 is an annular protrusion formed in order to increasethe uniformity of magnetic field distribution within the air gap 4, andanother known structure is one in which a tiered protrusion (not shown)is formed on the inside of the annular protrusion in an effort tofurther increase the uniformity of the field distribution.

In the figure, 6 is a tilt field coil, which is disposed in order toobtain information about positioning within the air gap 4. TheseGradient magnetic field coils 6 usually comprise a group of three coilscorresponding to the three directions X, Y, and Z within the air gap 4,but are shown in simplified form in the figure.

With a structure such as this, the air gap 4 must be large enough forall or part of a patient's body to be inserted therein, and a staticmagnetic field having a high uniformity of 1×10⁻⁴ or less at 0.02 to 2.0T must be formed within a specified image field of view within the airgap 4.

With the structure shown in FIGS. 13(a) and (b), a so-called four-columnyoke consisting of a pair of yoke plates 3 a and 3 b and four yokecolumns 3 c is used as the yoke 3, but as shown in FIGS. 14(a) and (b),variously structured yokes can be used according to the requiredcharacteristics, such as a so-called C yoke consisting of a pair of yokeplates 3 a and 3 b and a supporting yoke plate 3 d.

With the structure shown in FIGS. 13(a) and (b), permanent magnets suchas R-Fe-B-based magnets are employed as the field generation source, butother structures can also be used, such as one in which anelectromagnetic coil is wound around the periphery of an iron core.

Regardless of which of these structures is used, the air gap 4 is formedby the pair of pole pieces 2, and Gradient magnetic field coils 6 aredisposed in the vicinity of the pole pieces 2, as shown in FIGS. 13(a)and (b).

Usually, the pole pieces 2 are made from electromagnetic soft iron, pureiron, or another such bulk material (integrated), so when a pulsecurrent is passed through the Gradient magnetic field coils 6 and apulse-form tilt field is generated in the desired direction in order toobtain information about positioning within the air gap 4, the effect ofthis tilt field generates an eddy current in the pole pieces 2 anddecreases the rise characteristics of the tilt field, and even after theflow of the pulse current has been halted, the uniformity of the fielddistribution in the air gap 4 is decreased by the residual magnetismgenerated in the pole pieces 2.

As a means for solving this problem, MRI magnetic field generatorscharacterized in that the main portion of the pole pieces is formed fromlaminated silicon steel sheets have already been proposed by theinventors (Japanese Patent No. 2,649,436, Japanese Patent No. 2,649,437,U.S. Pat. No. 5,283,544, and European Patent No. 0479514).

The MRI magnetic field generators previously proposed by the inventorsare chiefly characterized by the use of pole pieces structured as shownin FIGS. 16 to 18.

The structure of the pole piece 10 shown in FIGS. 15(a) and (b)comprises a soft iron magnetic ring having a rectangular cross-sectionalshape and constituting an annular protrusion 12 on the air gap-facingside of a magnetic base member 11 composed of pure iron or another bulkmaterial, and a plurality of laminated blocks 13 produced by laminatinga plurality of silicon steel sheets in the facing direction of the polepieces and integrating these with an insulating adhesive agent or thelike.

In the figure, 14 is a tiered protrusion formed on the inside of theannular protrusion 12 for the purpose of enhancing the uniformity of thefield distribution. Just as discussed above, a plurality of siliconsteel sheets are laminated in the facing direction of the pole piecesand integrated with an insulating adhesive agent or the like, and theresulting plurality of laminated blocks are laminated in the requirednumber.

15 in the figure is a soft iron core used for mounting the fieldgeneration coil.

16 in the figure is a slit formed in the radial direction for thepurpose of dividing the soft iron magnetic ring having a rectangularcross-sectional shape and constituting the annular protrusion 12 into aplurality of sections in the circumferential direction and reducing theeddy current that is generated at the annular protrusion 12.

If the silicon steel sheets used in the above-mentioned laminated blocks13 are directional silicon steel sheets (JIS C 2553, etc.), then fromthe standpoint of field distribution uniformity, it is preferable forthem to be laminated and integrated such that the readily magnetizableaxis direction (calendering direction) is rotated by 90 degrees everyspecific number of small blocks 13 a and 13 b as shown in FIG. 16(a). Ifthe sheets are non-directional silicon steel sheets (JIS C 2552, etc.),then lamination and integration are performed merely in the thicknessdirection, without taking directionality into account, as shown in FIG.16(b).

The structure of the pole pieces 20 shown in FIGS. 17(a) and (b)comprises a soft iron magnetic ring having a rectangular cross-sectionalshape and constituting an annular protrusion 22 on the void-facing sideof a magnetic base member 21 composed of pure iron or another bulkmaterial, and a plurality of laminated blocks 23 produced by laminatinga plurality of non-directional silicon steel sheets in the directionperpendicular to the facing direction of the pole pieces and integratingthese with an insulating adhesive agent or the like.

In the figure, 24 is a tiered protrusion formed on the inside of theannular protrusion 22 for the purpose of enhancing the uniformity of thefield distribution, 25 is a soft iron core used for mounting the fieldgeneration coil, and 26 is a slit that divides the soft iron magneticring having a rectangular cross-sectional shape and constituting theannular protrusion 22 into a plurality of'sections in thecircumferential direction.

It is preferable for the above-mentioned laminated blocks 23 to belaminated and integrated with an insulating adhesive agent or the likesuch that the lamination direction is rotated by 90 degrees for everyone of the small blocks 23 a and 23 b laminated in the void-facingdirection, as shown in FIG. 17(c).

The structure of the pole pieces 30 shown in FIGS. 18(a) and (b) isquite different from that of the pole pieces 10 and 20 shown in FIGS. 15(a) and (b) and FIGS. 17(a) and (b), respectively, in that the magneticbase members 11 and 21 composed of a bulk material are not used.Specifically, this structure is such that, instead of the magnetic basemembers 11 and 21 composed of a bulk material, laminated rods 33,produced by laminating a plurality of non-directional silicon steelsheets, as shown in FIG. 18(c), in the direction perpendicular to thefacing direction of the pole pieces and laminating these with aninsulating adhesive agent or the like, are supported by an annularsupport member 34 composed of a bulk magnetic material.

To discuss this in more detail, the center portion of the annularsupport member 34 composed of a bulk magnetic material is cut out, andthe above-mentioned laminated rods 33 a are disposed unidirectionallysuspended therein with the chamfers 38 thereof corresponding to thechamfers (not shown) formed around the inside edges of the cutout. Thelaminated rods 33 b are laid out as a second layer such that thelamination direction is rotated by 90 degrees on the void-facing side ofthe laminated rods 33 a.

A plurality of laminated rods 33 c of different length are disposedbetween a fixed plate 35 and the inner peripheral surface of the annularsupport member 34 so that the overall shape of the pole piece willapproximate that of a disk, and a soft iron magnetic ring having atrapezoidal cross section and constituting the annular protrusion 32 isinstalled via fixed blocks 31 fixed at specific positions around theoutside edge of the inner periphery of the annular support member 34,forming the pole piece 30.

36 in the figure is a slit that divides the soft iron magnetic ringhaving a trapezoidal cross-sectional shape and constituting the annularprotrusion 32 into a plurality of sections in the circumferentialdirection. 37 is an insulating material composed of an insulatingadhesive tape or the like.

By using the pole pieces 10, 20, and 30 shown in FIGS. 15(a) and (b),17(a) and (b), and 18(a) and (b) as above, is it possible to greatlyreduce the generation of residual magnetism and eddy current in the polepieces that is caused by the Gradient magnetic field coils as comparedto when the conventional pole pieces composed of a bulk magneticmaterial shown in FIGS. 13(a) and (b) and 14(a) and (b) are used.

DISCLOSURE OF THE INVENTION

However, there is a growing need for an MRI magnetic field generatorcapable of producing sharp images at even higher speed, and furtherimprovement is desired.

It has been confirmed in experiments conducted by the inventors that thestructures of the above-mentioned pole pieces 10 and 20 in FIGS. 15(a)and (b) and 17(a) and (b) have numerous advantages, such as producingexcellent mechanical strength (rigidity) for the pole piece as a wholebecause of the use of the magnetic base members 11 and 12 composed of abulk material, and affording easy assembly work because of how easy itis to laminate and lay out the plurality of laminated blocks 13 and 23produced by laminating silicon steel sheets in a specific direction andintegrating these with an insulating adhesive agent or the like.Nevertheless, the very presence of these magnetic base members 11 and 21prevents any further reduction in the residual magnetism and eddycurrent in the pole pieces.

Specifically, it has been confirmed that the magnetic field generated bythe Gradient magnetic field coils goes from the laminated blocks 13 and23 directly under the Gradient magnetic field coils, through themagnetic base members 11 and 21 on which these laminated blocks 13 and23 are placed, and reaches the surface of the soft iron magnetic ringthat constitutes the annular projection 12 and 22. Therefore, themagnetic base members 11 and 21 end up being present along the magneticpath between the laminated blocks 13 and 23 and the soft iron magneticring, and as a result an eddy current and residual magnetism aregenerated within the magnetic base members 11 and 21 composed of a bulkmaterial.

With the pole pieces 30 structured as in FIGS. 18(a) and (b), theeffective use of the annular support member 34 affords the sameexcellent mechanical strength and ease of assembly as the structures ofthe pole pieces 10 and 20 in FIGS. 15(a) and (b) and 17(a) and (b).

With this structure, no magnetic base members 11 and 21 composed of abulk material, such as those used for the pole pieces 10 and 20 in FIGS.15(a) and (b) and 17(a) and (b), are present under the laminated rods 33directly beneath the Gradient magnetic field coils, which is preferablefrom the standpoint of reducing eddy current and residual magnetism, butbecause the annular support member 34 present under the laminated rods33 is also composed of a bulk magnetic material, the result is that therequired reduction in eddy current and residual magnetism cannotnecessarily be achieved at the present time.

Furthermore, the void 39 is formed without the inner peripheral surfaceof the annular support member 34 being in complete contact with thelaminated rods 33 c, and as a result, the ratio (Sb/Sa) between theoverall surface area Sa on the side of annular protrusion 32 facing thelaminated silicon steel sheets and the overall surface area Sb on theside of the laminated silicon steel sheets facing the annular protrusion32 is less than 80% (about 70 to 75%), resulting in a magneticallyunsaturated state occurring where the annular protrusion 32 meets thelaminated silicon steel sheets, and this sometimes impedes the flow ofthe magnetic flux to the annular protrusion 32 and makes it difficult toefficiently obtain a specific uniform magnetic field within the air gapbetween the pole pieces.

Specifically, the flux density produced by the magnetic field from thefield generation source is far higher where the annular protrusion 32meets the laminated silicon steel sheets than in other portions, and thelaminated silicon steel sheets in contact with the annular protrusion 32in particular need to have enough volume to avoid a magneticallyunsaturated state. The inventors have confirmed, however, that theuniform magnetic field originally required for an MRI magnetic fieldgenerator cannot be obtained with the structure shown in FIGS. 18(a) and(b).

It is an object of the present invention to provide an MRI magneticfield generator that solves the above problems, and it is a furtherobject to provide an MRI magnetic field generator with which it ispossible to lower the residual magnetism and eddy current within polepieces generated by the effect of the pulse current flowing throughGradient magnetic field coils, without decreasing the field uniformitywithin the air gap.

The inventors perfected the invention upon learning that the statedobject can be effectively achieved by optimizing the disposition of thelaminate of silicon steel sheets.

Specifically, the present invention is an MRI magnetic field generatorthat has a pair of pole pieces facing each other so as to form a air gapand that generates a magnetic field in this air gap, wherein the polepieces each comprise a main component consisting of laminated siliconsteel sheets, and a magnetic annular protrusion disposed on the side ofthe main component facing the air gap.

The inventors also propose as favorable structures a structure in whichthe main component consisting of laminated silicon steel sheets is fixedand supported by a non-magnetic support member with high electricalresistance; a structure in which the main component is fixed andsupported by a magnetic annular support member divided into a pluralityof sections in the circunferential direction; a structure in which themagnetic annular protrusion consists of laminated silicon steel sheetsin order to reduce the eddy current generated in this annularprotrusion; a structure in which the annular protrusion is divided intoa plurality of sections in the circumferential direction; a structure inwhich a tiered protrusion comprising laminated silicon steel sheets isformed on the inside of the magnetic annular protrusion and on the sideof the pole piece main component facing the air gap; and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the structure of the MRI magnetic fieldgenerator pertaining to the present invention, with (a) being a verticalcross section, (b) a top view, and (c) an oblique view of the maincomponent;

FIG. 2 is an oblique view of the main component, illustrating thestructure of the MRI magnetic field generator pertaining to the presentinvention;

FIG. 3 is a diagram illustrating another structure of the MRI magneticfield generator pertaining to the present invention, with (a) being avertical cross section, (b) a top view, and (c) an oblique view of themain component;

FIG. 4 is a diagram illustrating another structure of the MRI magneticfield generator pertaining to the present invention, with (a) being avertical cross section, (b) a top view, and (c) an oblique view of themain component;

FIG. 5 is a diagram illustrating another structure of the MRI magneticfield generator pertaining to the present invention, with (a) being avertical cross section, (b) a top view, and (c) an oblique view of themain component;

FIG. 6 is a diagram illustrating another structure of the MRI magneticfield generator pertaining to the present invention, with (a) being avertical cross section, (b) a top view, and (c) an oblique view of themain component;

FIGS. 7(a), (b), (c), (d), and (e) are oblique views of the structure ofthe magnetic annular protrusion of the MRI magnetic field generatorpertaining to the present invention;

FIG. 8 is a vertical cross section illustrating a detail view of the MRImagnetic field generator pertaining to the present invention;

FIG. 9 is a vertical cross section illustrating a detail view of the MRImagnetic field generator pertaining to the present invention;

FIG. 10(a) is a vertical cross section illustrating a detail view of theMRI magnetic field generator pertaining to the present invention, and(b) is a top view;

FIGS. 11(a) and (b) are vertical cross sections illustrating detailviews of the MRI magnetic field generator pertaining to the presentinvention;

FIG. 12 is a graph of the relationship between magnetic field uniformityand distance from the center in the air gap;

FIG. 13 is a diagram illustrating the structure of a conventional MRImagnetic field generator, with (a) being a front view and (b) a lateralcross section;

FIG. 14 is a diagram illustrating another structure of a conventionalMRI magnetic field generator, with (a) being a front view and (b) alateral cross section;

FIG. 15 is a diagram illustrating another structure of a conventionalMRI magnetic field generator, with (a) being a vertical cross sectionand (b) a front view;

FIGS. 16(a) and (b) are oblique views of the laminated blocks used in aconventional URI magnetic field generator;

FIG. 17 is a diagram illustrating another structure of a conventionalMRI magnetic field generator, with (a) being a vertical cross section,(b) a front view, and (c) an oblique view of the laminated blocks; and

FIG. 18 is a diagram illustrating another structure of a conventionalMRI magnetic field generator, with (a) being a vertical cross section,(b) a front view, and (c) an oblique view of the laminated rods.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described on the basis of the examplesillustrated in FIGS. 1 to 12.

The pole piece 40 pertaining to the present invention and illustrated inFIGS. 1(a), (b), and (c) has as its main constituent members a maincomponent 41 consisting of a plurality of laminated blocks fixed andsupported by a non-magnetic annular support member 43 with highelectrical resistance and produced by laminating a plurality of siliconsteel sheets in the facing direction of the pole pieces and integratingthese with an insulating adhesive agent or the like, and a magneticannular protrusion 42 with a rectangular cross section and placed on theside of this main component 41 facing the air gap.

For the main component 41, a plurality of laminated blocks consisting ofsilicon steel sheets are first integrated into a rectangular sheetshape, after which the outer periphery is worked into a specific shapeby water jet working, laser working, machining, discharge working, orthe like so that the overall shape approximates that of a disk. The maincomponent 41 is fixed and supported by disposing a non-magnetic annularsupport member 43 with high electrical resistance and composed of aresin, bakelite, FRP, or another such non-metal around its peripheraledge. It is also possible to ensure good mechanical strength by using anepoxy resin or the like to mold the plurality of laminated blocks andthe non-magnetic annular support member 43.

Another option, for the purpose of increasing the uniformity of thefield distribution in the air gap, is to provide a tiered protrusion 44consisting of laminated silicon steel sheets that have been worked intothe above-mentioned approximate disk shape and formed on the inside ofthe magnetic annular protrusion 42 and on the side of the laminatedsilicon steel sheets facing the air gap. This tiered protrusion 44 isalso constituted by a plurality of laminated blocks produced bylaminating a plurality of silicon steel sheets in the facing directionof the pole pieces and integrating these with an insulating adhesiveagent or the like.

In the figure, the tiered protrusion 44 is structured such that theabove-mentioned laminated blocks are laid over the entire surface on theinside of the magnetic annular protrusion 42, and the thickness of thecenter portion in the pole piece facing direction in particular isincreased to achieve an overall tiered shape, but it is also effectivefor the laminated blocks to be laid only in the center portion, and notprovided near the inside of the magnetic annular protrusion 42.

In the present invention, the “main component consisting of laminatedsilicon steel sheets” refers to the portion facing the tilt field coil,including the tiered protrusion 44. The provision of a tiered protrusionwill also be described for the structure of the pole pieces describedbelow, but this tiered protrusion is not essential in the presentinvention. Specifically, other structures may also be employed toachieve an increase in uniformity of the field distribution within theair gap rather than providing a tiered protrusion, such as disposingyokes, permanent magnets, or the like at specific positions on the side(flat side) of the above-mentioned approximately disk-shaped Laminatedsilicon steel sheets facing the air gap.

FIG. 1(c) is an oblique view illustrating the relationship of thelamination direction of the laminated blocks of silicon steel sheetsthat constitute the main component 41. As previously described, if thesilicon steel sheets used in these laminated blocks are directionalsilicon steel sheets (JIS C 2553, etc.), then from the standpoint offield distribution uniformity, it is preferable for them to be laminatedand integrated such that the readily magnetizable axis direction(calendering direction) is rotated by 90 degrees every specific numberof small blocks as shown in FIG. 16(a). In the case of non-directionalsilicon steel sheets (JIS C 2552, etc.), then lamination and integrationare performed merely in the thickness direction, without takingdirectionality into account, as shown in FIG. 16(b).

As shown in FIG. 2, a further reduction in the effect of residualmagnetism or eddy current due to the Gradient magnetic field coils canbe achieved by disposing connecting end (side) faces 41 a and 41 b ofthe laminated blocks of adjacent silicon steel sheets so that thesefaces do not line up in the lamination direction of the variouslaminates.

Specifically, if there is leakage of the magnetic field generated by theGradient magnetic field coils from the gaps that are inevitably formedat the connecting end faces of the various laminated blocks, and if thisleaked magnetic field acts on the permanent magnet structure that servesas the field generation source, then an eddy current, albeit a veryslight one, will be generated on the permanent magnet structure surface,and the field distribution uniformity within the air gap will becomeunstable due to heat generation and the like caused by this eddycurrent.

However, as shown in FIG. 2, if the various connecting end faces of thelaminated blocks of silicon steel sheets are disposed so that they donot line up, then a magnetic path will be formed within the air gapwithout a substantial increase in magnetic permeability (decrease inmagneto resistance) in the direction perpendicular to the laminationdirection of the laminated blocks (i.e., the horizontal direction in thefigure), and without the magnetic field generated by the Gradientmagnetic field coils infiltrating the permanent magnet structure.

Similarly, with the pole pieces described below, it is favorable toemploy the same structure for the disposition of the various connectingend faces of the laminated blocks of silicon steel sheets.

In the figure, 46 is a slit that divides the soft iron magnetic ringhaving a rectangular cross section and constituting the magnetic annularprotrusion 42 into a plurality of sections in the circumferentialdirection, and that is formed in the radial direction for the purpose ofreducing the eddy current generated within the magnetic annularprotrusion 42.

The relationship between the main component 41 and the magnetic annularprotrusion 42 in the above structure will be described in detail throughreference to FIG. 8. The laminated silicon steel sheets that constitutethe main component 41 are formed in an approximate disk shape, and theoutside diameter D₀ thereof is roughly the same as the outside diameterD₁ of the magnetic annular protrusion 42. Therefore, the side of themagnetic annular protrusion 42 facing the main component 41 is incontact with the laminated silicon steel sheets.

It is therefore possible for the originally required field intensity tobe efficiently generated in the specified air gap of the MRI magneticfield generator without the magnetic flux produced by the magnetic fieldfrom the permanent magnet structure 1 serving as the field generationsource leading to a magnetically unsaturated state where the laminatedsilicon steel sheets and the magnetic annular protrusion 42 touch.

Also, in the main component 41, the bulk magnetic material is notdisposed either directly below or to the inside of the inside diameterD₂ of the magnetic annular protrusion 42. Specifically, since no bulkmagnetic material is disposed in the vicinity of directly below theGradient magnetic field coils, it is possible to obtain the desiredreduction in eddy current and residual magnetism.

The outside diameter D₀ of the laminated silicon steel sheetsconstituting the main component 41 is not limited to being the same asthe outside diameter D₁ of the magnetic annular protrusion 42 as shownin the figure. For instance, it is also possible for this diameter D₀ tobe larger than the outside diameter D₁ of the magnetic annularprotrusion 42, but making it unnecessarily large is undesirable becausethis will increase leakage of the magnetic flux from the outer peripheryof the laminated silicon steel sheets.

It is also possible for the outside diameter D₀ of the laminated siliconsteel sheets to be smaller than the outside diameter D₁ of the magneticannular protrusion 42, but making it unnecessarily small will lead to amagnetically unsaturated state where the laminated silicon steel sheetsand the magnetic annular protrusion 42 touch, so it is preferable forthe ratio (Sb/Sa) between the overall surface area Sa on at least theside of the magnetic annular protrusion 42 facing the laminated siliconsteel sheets and the overall surface area Sb on the side of thelaminated silicon steel sheets facing the magnetic annular protrusion 42to be at least 80%, with 85% or more being preferable, and 90% or morebeing even better. The figure illustrates a case of 100% (Sa=Sb).

The figure illustrates a structure in which the non-magnetic annularsupport member 43 is disposed on the outside of the magnetic annularprotrusion 42, but if, as mentioned above, the outside diameter D₀ ofthe laminated silicon steel sheets constituting the main component 41 ismade smaller than the outside diameter D₁ of the magnetic annularprotrusion 42, then all or part of the non-magnetic annular supportmember 43 will naturally end up being disposed directly beneath themagnetic annular protrusion 42.

However, it will be possible to obtain the desired effect from thestandpoint of reduced eddy current and residual magnetism if thematerial of the non-magnetic annular support member 43 is selected fromamong a resin, bakelite, FRP, or another such non-metal and has highelectrical resistance.

In terms of the magnetic annular protrusion 52 and so forth, the polepiece 50 pertaining to the present invention and shown in FIGS. 3(a),(b), and (c) has the same structure as that shown in FIGS. 1(a), (b),and (c), except for the main component 51. Specifically, the pole piece50 pertaining to the present invention and shown in FIGS. 3(a), (b), and(c) is formed such that a plurality of approximately disk-shapedlaminated silicon steel sheets, which are fixed and supported by anon-magnetic annular support member 53 and constitute the main component61, are laminated in the facing direction of the pole pieces andintegrated with an insulating adhesive agent or the like. FIG. 3(c) isan oblique view illustrating the relationship of the laminationdirection of the silicon steel sheets that constitute the main component51, including a tiered protrusion 54.

With this structure, except for the tiered protrusion 64, there is noneed to combine a plurality of laminated blocks as shown in FIG. 1, sonot only is production easier, but mechanical strength is also better.Also, in order to further enhance the reduction in eddy current andresidual magnetism, it is preferable for the laminated silicon steelsheets composed of this disk-shaped sheet material to be divided in thecircumferential direction, and for semi-circular, pie-shaped, or othersuch laminates to be combined.

In terms of the magnetic annular protrusion 62 and so forth, the polepiece 60 pertaining to the present invention and shown in FIGS. 4(a),(b), and (c) has the same structure as that shown in FIGS. 1(a), (b),and (c), except for the main component 61. Specifically, with the polepiece 60 pertaining to the present invention and shown in FIGS. 4(a),(b), and (c), an approximately disk-shaped laminate of silicon steelsheets, which is fixed and supported by a non-magnetic annular supportmember 63 and constitutes the main component 61, is formed by thedisposition of a plurality of laminated blocks produced by laminating aplurality of silicon steel sheets in the direction perpendicular to thefacing direction of the pole pieces and integrating these with aninsulating adhesive agent or the like.

It is preferable, both from the standpoint of magnetic field uniformityand from the standpoint of mechanical strength, for the main component61 to be such that the lamination direction is rotated by 90 degrees forevery one of the small blocks that are laminated in the void-facingdirection, as previously described for FIG. 17(c), and such that thelamination direction is rotated by 90 degrees for every one of thelaminated blocks adjacent in the same plane. FIG. 4(c) is an obliqueview illustrating the relationship of the lamination direction of thesilicon steel sheets constituting the main component 61, including thetiered protrusion 64.

In terms of the magnetic annular protrusion 72 and so forth, the polepiece 70 pertaining to the present invention and shown in FIGS. 5(a),(b), and (c) has the same structure as that shown in FIGS. 1(a), (b),and (c), except for the main component 71. Specifically, with the polepiece 70 pertaining to the present invention and shown in FIGS. 5(a),(b), and (c), an approximately disk-shaped laminate of silicon steelsheets, which is fixed and supported by a non-magnetic annular supportmember 73 and constitutes the main component 71, is formed by thedisposition of a plurality of laminated blocks produced by laminating aplurality of silicon steel bands in the direction perpendicular to thefacing direction of the pole pieces and integrating these with aninsulating adhesive agent or the like. FIG. 5(c) is an oblique viewillustrating the relationship of the lamination direction of the siliconsteel sheets constituting the main component 71, including the tieredprotrusion 74.

In one method that can be employed to facilitate assembly and increasemechanical strength, a non-magnetic annular support member 73 in which asupport 73 a for the silicon steel sheets is provided to the edge on theinner periphery is readied, and strips of silicon steel sheets ofvarying length and having chamfers 71 a formed corresponding to theshape of this support 73 a are successively laid out and suspended, orare laid out and suspended after first being laminated for every fewsheets and integrated into laminated rods.

It is also possible to laminate and integrate a plurality of strips ofsilicon steel sheets in one direction to produce a laminate ofrectangular sheets, work this laminate into an approximate disk shape bywater jet working or the like, and form a chamfer 71 a corresponding tothe shape of the support 73 a of the non-magnetic annular support member73 around the edge of this laminate.

The silicon steel sheets used here may be either directional siliconsteel sheets or non-directional silicon steel sheets, but the use ofnon-directional silicon steel sheets is preferable from the standpointof ease of manufacture. For example, there is no need to takedirectionality into account in the cutting of the various strips ofsilicon steel sheets that constitute the laminate into the requiredshapes.

In terms of the magnetic annular protrusion 82 and so forth, the polepiece 80 pertaining to the present invention and shown in FIGS. 6(a),(b), and (c) has the same structure as that shown in FIGS. 1(a), (b),and (c), except for the tiered protrusion 84. Specifically, with thepole piece 80 pertaining to the present invention and shown in FIGS.6(a), (b), and (c), the tiered protrusion 84 formed on the side of themain component 81 facing the air gap has a structure in which there aredisposed a plurality of laminated blocks produced by laminating aplurality of silicon steel sheets in the direction perpendicular to thefacing direction of the pole pieces and then integrating these with aninsulating adhesive agent or the like. FIG. 6(c) is an oblique viewillustrating the relationship of the lamination direction of the siliconsteel sheets constituting the main component 81, including the tieredprotrusion 84.

It is preferable, both from the standpoint of magnetic field uniformityand from the standpoint of mechanical strength, for the approximatelydisk-shaped laminated silicon steel sheets that constitute the maincomponent 81 to be disposed such that the lamination direction isrotated by 90 degrees for every one of the small blocks that arelaminated in the void-facing direction, as previously described for FIG.17(c), and such that the lamination direction is rotated by 90 degreesfor every one of the laminated blocks adjacent in the same plane. 83 inthe figure is a non-magnetic annular support member.

The silicon steel sheets constituting the tiered protrusion 84 may beeither directional silicon steel sheets or non-directional silicon steelsheets, but the use of non-directional silicon steel sheets ispreferable from the standpoint of ease of manufacture. For example,there is no need to take directionality into account in the cutting ofthe various pieces of silicon steel sheets that constitute the laminatedblocks into the required shapes.

The pole pieces 50, 60, 70, and 80 pertaining to the present inventionand described above are all the same as the pole pieces 40 pertaining tothe present invention and shown in FIGS. 1(a), (b), and (c) in terms ofthe relationship between the main components 51, 61, 71, and 81 to themagnetic annular projection 52, 62, 72, and 82, so it is possible forthe originally required field intensity to be efficiently generated inthe specified air gap of the MRI magnetic field generator, andfurthermore it is possible to achieve a reduction in eddy current andresidual magnetism, without leading to a magnetically unsaturated statewhere the main components 51, 61, 71, and 81 and the magnetic annularprojection 52, 62, 72, and 82 touch.

It has already been mentioned that it will be possible to obtain thedesired effect from the standpoint of reduced eddy current and residualmagnetism even if all or part of the non-magnetic annular support memberis disposed directly beneath the magnetic annular protrusion with theabove structure since the annular support member is made from a materialwith high electrical resistance, composed of a resin, bakelite, FRP, oranother such non-metal.

However, as shown in FIG. 10(a), from the standpoints of mechanicalstrength, workability, and so forth, the effect of reducing eddy currentand residual magnetism is greatly diminished if all or part of amagnetic annular support member 93 is disposed directly beneath themagnetic annular protrusion 92 when soft iron or another such magneticmaterial is used for the annular support member that fixes and supportsthe main component 91.

The inventors have confirmed experimentally that this diminishment ofthe effect of reducing eddy current and residual magnetism can besuppressed by dividing the magnetic annular support member 93 into aplurality of sections, as shown in FIG. 10(b) (the figure shows a caseof eight), in the circumferential direction with slits 96.

When the magnetic annular support member 93 is used, unlike when anon-magnetic annular support member was used, the magnetic annularsupport member 93 serves as a member that forms a magnetic path overwhich the magnetic flux produced by the magnetic field is transmittedfrom the permanent magnet structure 1 (the field generation source) tothe annular protrusion, and it is therefore possible for the originallyrequired field intensity to be efficiently generated in the specifiedair gap of the MRI magnetic field generator without the magnetic fluxleading to a magnetically unsaturated state where the laminated siliconsteel sheets and the magnetic annular protrusion touch.

Therefore, if we take into account the overall mechanical strength ofthe pole pieces, some of the factors for which are the shape and sizeand the lamination structure of the silicon steel sheets constitutingthe main component, as well as the effect of reducing eddy current andresidual magnetism, and the magnetically unsaturated state where themain component and the magnetic annular protrusion touch, it ispreferable for the annular support member to be made of a materialselected from either a non-magnetic material with high electricalresistance and composed of a resin, bakelite, FRP, or another suchnon-metal, or a magnetic material such as soft iron, and it isparticularly favorable to employ a structure in which this annularsupport member is divided into a plurality of sections in thecircumferential direction as discussed above.

It is also possible to use a non-magnetic material composed of aluminum,copper, stainless steel, or another metal instead of the soft ironsupport member, but because these materials also have low electricalresistance, it is preferable to employ a structure in which there are aplurality of sections divided in the circumferential direction, just aswith a soft iron support member.

For all of the above structures, the description was of one in which anannular support member was used, but the decision as to whether anannular support member is needed should be made according to the overallshape and size of the pole pieces, the shapes and sizes of the variouslaminated blocks of silicon steel sheets, their mutual adhesivestrength, the mechanical strength of the pole pieces, which is naturallydetermined according to the integration method, that is, a methodinvolving molding the entire pole piece from a resin, a method involvingweaving in metal fibers, a method involving first placing andintegrating on the permanent magnet structure or the like that willserve as the field generation source and then removing the jig, and soon, depending on whether the desired shape can be maintained by theattractive force from the field generation source when the fieldgeneration source is a permanent magnet structure, for example. It isfurther possible to use the above-mentioned non-magnetic annular supportmember together with a magnetic annular support member, in which casethe shape of each is not limited to the structures shown in the figures.

The various support members, particularly when placed on the fieldgeneration source, are effective at ensuring the above-mentionedmechanical strength required of the pole pieces, but they are notnecessarily required as long as the assembly of the magnetic fieldgenerator placed on the field generation source can be completed and thedesired pole piece shape can be maintained, and when the relationshipwith other devices and the like disposed around the pole pieces is takeninto account, these support members may be removed upon completion ofthe assembly. Also, in addition to whether or not there is an annularsupport member, the outer periphery of the main component does notnecessarily have to be worked into an approximate disk shape, dependingon the method for integrating the laminated blocks of silicon steelsheets.

In all of the above structures, a bulk magnetic material with arectangular cross section was used as the annular protrusion placed onthe side of the main component facing the air gap, but reducing the eddycurrent and residual magnetism in the annular protrusion is alsoeffective in order to satisfy all of the characteristics required of anMRI magnetic field generator, and it is favorable for the entire annularprotrusion, or just the surface layer thereof, to be constituted by alaminate of silicon steel sheets, as shown in FIG. 7. FIG. 7 is anoblique view of part of an annular protrusion divided in sections in thecircumferential direction.

Specifically, FIG. 7(a) is the magnetic annular protrusion 42 in whichonly a bulk magnetic material with a rectangular cross section was used,whereas in FIG. 7(b) a bulk magnetic material 42 a with a rectangularcross section is used as a core, with a laminate of silicon steel sheets42 b disposed around the periphery thereof, that is, laminated on theside facing the air gap and on the inner peripheral surface, in thedirection facing the pole piece, so that only the surface layer of theannular protrusion is a laminate of silicon steel sheets.

In FIG. 7(c), the entire annular protrusion is constituted by a laminateof silicon steel sheets 42 c laminated in the direction facing the polepiece. In FIG. 7(d), a bulk magnetic material 42 a with a rectangularcross section is used as a core, with a laminate of silicon steel sheets42 d disposed around the periphery thereof (the side facing the air gapand on the inner peripheral surface), laminated in the directionperpendicular to the direction facing the pole piece. In FIG. 7(e), theentire annular protrusion is constituted by a laminate of silicon steelsheets 42 e, laminated concentrically and in the direction perpendicularto the direction facing the pole piece.

In FIG. 9, a magnetic annular protrusion 42 with the structure in FIG. 1is changed to the structure in FIG. 7(c) discussed above, which makespossible a reduction in eddy current and residual magnetism in thismagnetic annular protrusion 42, and allows the characteristics of theoverall pole piece to be greatly enhanced.

With this structure, since the magnetic annular protrusion 42 is alsoformed from a laminate of silicon steel sheets, the mechanical strengthof the pole pieces overall is somewhat inferior to that of the structurein FIGS. 1(a), (b), and (c). Therefore, it is possible to enhance themechanical strength of the pole pieces overall by disposing thenon-magnetic annular support members 43 a and 43 b around the peripheraledges of the laminated silicon steel sheets constituting the maincomponent 41 and around the peripheral edge of the magnetic annularprotrusion 42, then disposing the non-magnetic annular support member 43c so as to integrally enclose these, and then performing resin molding.

Also, because the pole pieces constituting the magnetic field generatorof the present invention are substantially constituted by a plurality oflaminated blocks of silicon steel sheets, in order to increase themechanical strength of the pole pieces overall, it is also effective toadhesively fix a magnetic lamina 47 to the bottoms of the pole pieces,that is, to the side of the main component 41 composed of a laminate ofsilicon steel sheets that is opposite the side facing the air gap, asshown in FIG. 11(a), although this will somewhat diminish the originaleffect of reducing eddy current and residual magnetism.

This magnetic lamina 47 makes it possible to increase the bonded surfacearea of the individual laminated blocks of silicon steel sheets andprevent their lateral shift respective to one another. However, thedesired effect of reducing eddy current and residual magnetism will notbe obtained if this magnetic lamina 47 is thicker than necessary.

It is therefore preferable for the magnetic lamina 47 to be extremelythin, and should be no more than 10% of the thickness of the maincomponent comprising a laminate of silicon steel sheets (when a tieredprotrusion is provided, the thickness including this tiered protrusion).The mechanical strength of the pole pieces can be ensured by setting thethickness of the magnetic lamina at no more than 3% of the thickness ofthe main component, but a thickness of about 5% is also effective fromthe standpoint of handling when we consider such factors as integrationwith the magnetic annular protrusion and other members with screws.

Again when the above magnetic lamina is used, it is preferable from thestandpoint of reducing eddy current and residual magnetism to divide themagnetic lamina into a plurality of sections in the circumferentialdirection and combine these semi-circular, pie-shaped, or othersections.

In order to integrate the magnetic annular protrusion and the maincomponent with screws, as shown in FIG. 11(b), a flat ring-shapedmagnetic plate 48 (no more than 20%, and preferably no more than 15%, ofthe thickness of the main component 41) divided into a plurality ofsections in the circumferential direction may be disposed at a positioncorresponding to the peripheral edge of the magnetic lamina 47 directlybeneath the magnetic annular protrusion 42, which is relativelysusceptible to the effect of the magnetic field generated by theGradient magnetic field coils.

Furthermore, it is also possible to employ a structure in which anon-magnetic lamina with high electrical resistance, such as a resin,bakelite, or FRP, in place of the above-mentioned magnetic lamina 47.This disposition of a non-magnetic lamina will not result in thegeneration of an eddy current or residual magnetism, and while thethickness thereof can be selected as desired, excessive thickness willprevent the efficient formation of the magnetic field generated from thefield generation source within the air gap.

Therefore, when a non-magnetic lamina is disposed in a structure inwhich a permanent magnet structure is used as the field generationsource, it is favorable for the permanent magnets to have highermagnetic characteristics than usual, or for the volume of the permanentmagnet structure to be increased somewhat.

Also, with a structure in which an electromagnetic coil is used as thefield generation source, it is preferable for the current applied to theelectromagnetic coil to be increased somewhat.

The MRI magnetic field generator of the present invention is not limitedto a structure in which permanent magnets such as R-Fe-B-based magnetsare used as the field generation source, and also encompasses structuressuch as one in which an electromagnetic coil (including normalconduction coils, superconduction coils, and so on) is wound around aniron core, but in order for the advantages of constituting the maincomponent of the pole pieces from a laminate of silicon steel sheets tobe utilized most effectively, it is preferable to use a structure inwhich permanent magnets with a substantially high electrical resistanceand low magnetic permeability are employed.

Specifically, a preferred structure is one in which the main componentof the pole pieces is placed on the permanent magnet structure servingas the field generation source. Also, with a structure in which anon-magnetic lamina with high electrical resistance is disposed on thesurface of the pole piece main component on the side opposite the sidefacing the air gap, the advantages of the pole piece structure of thepresent invention can be effectively utilized even when anelectromagnetic coil is wound around an iron core.

In addition, with the magnetic field generator of the present invention,it is possible to employ known technology as needed, without beingrestricted to the structures illustrated in the figures.

EMBODIMENTS Embodiment 1

To confirm the effect of the MRI magnetic field generator of the presentinvention, its effect of reducing eddy current and residual magnetismwas evaluated by the following method. A tilt field coil was installedon various types of pole piece, a pulse current (500 AT) consisting of aspecific pulse (1 msec, 3 msec, or 5 msec) was applied to the fieldcoil, and the magnitude of the residual magnetism was measured with amilligauss meter. The measurement results are given in Tables 1 and 2.

The pole piece main component had an outside diameter of 1000 mm and athickness of 60 mm (maximum thickness including the tiered protrusion).The annular protrusion had an outside diameter of 1000 mm, and insidediameter of 920 mm, and a thickness of 50 mm, and was installed aroundthe peripheral edge of the pole piece main component. An FRP (glassfiber-reinforced plastic) support member had an outside diameter of 1040mm, an inside diameter of 1000 mm, and a thickness of 50 mm, and an ironsupport member had an outside diameter of 1000 mm, an inside diameter of960 mm, and a thickness of 50 mm.

Because it is usually extremely difficult to measure the eddy currentitself flowing through a metal, the magnitude of the eddy current wasevaluated from the amount of change in residual magnetism when the pulsewidth was varied. The values in the table indicate the differencebetween the residual magnetism at 1 msec and the residual magnetism at 5msec. Specifically, a short pulse width corresponds to a high frequency,whereas a long pulse width corresponds to a low frequency. The fact thatthe amount of residual magnetism changed with the pulse width means thatthere is a dependence on frequency. Therefore, if there is a largeamount of change, there is also a large eddy current, the result ofwhich is that a sharp image cannot be obtained.

The pole pieces A to E of the present invention can be seen to have lessresidual magnetism at all pulse widths, and to have less change in theresidual magnetism, than pole pieces with the conventional structureshown in FIG. 15, that is, a structure in which a magnetic base membercomposed of bulk iron with a thickness of 30 mm is disposed in the polepiece main component. In particular, pole piece B, in which FRP was usedas the support member and the annular protrusion comprised laminatedsilicon steel sheets, had a lower value for residual magnetism than theother structures and had an extremely small amount of change in residualmagnetism, so it can be seen that the effect of reducing eddy currentand residual magnetism is extremely good here.

When iron was used as a support member, and when this support member wasin a so-called bulk form and not divided (reference example), even whena laminate of silicon steel sheets was used for the pole piece maincomponent, the effect thereof could not be effectively utilized, and itcan be seen that the effect of the iron support member positioneddirectly beneath the annular protrusion resulted in residual magnetismand eddy current that were either the same as or worse than those in aconventional structure.

However, the effect of using a laminate of silicon steel sheets for thepole piece main component can be effectively realized by dividing thesupport member, as shown by pole piece C, or by using laminated siliconsteel sheets as the annular protrusion, as shown by pole piece D.

Pole piece E had a structure in which laminated silicon steel sheetswere used as the annular protrusion in addition to the structure shownin FIG. 3, and while the effect of reducing residual magnetism and eddycurrent was somewhat lower than with the other pole pieces A to D, itwas still clearly superior to that of the pole piece with a conventionalstructure. An effect equal to or better than the effect shown for polepiece E is obtained with all of the structures of the present inventionshown in FIGS. 4 to 6.

When the same measurements were made for a structure in which a magneticlamina with a thickness of 1.5 mm was further used with pole piece B(see FIG. 11), the magnetic lamina had almost no effect on eddy currentand residual magnetism, and the measurement results were about the sameas those for pole piece B.

TABLE 1 Annular projection Support member Structure Divisions inDivisions in reference circumferential circumferential figure Materialdirection Material direction Conventional example Comparison FIG. 15iron 8 divisions none — FIG. 10 iron 8 divisions iron none Presentinvention A FIG. 1 iron 8 divisions FRP — B FIG. 9 laminated 8 divisionsFRP — silicon steel sheets C FIG. 10 iron 8 divisions iron 8 divisions DFIG. 10 laminated 8 divisions iron 8 divisions silicon steel sheets EFIG. 3 laminated 8 divisions FRP — silicon steel sheets

TABLE 2 Structure Residual magnetism Eddy reference measurement resultscurrent figure 1 msec 3 msec 5 msec evaluation* Conventional exampleComparison FIG. 15 95 63 49 46 FIG. 10 101 60 48 53 Present invention AFIG. 1 36 26 24 12 B FIG. 9 22 21 20 2 C FIG. 10 54 29 22 32 D FIG. 1039 27 23 16 E FIG. 3 75 52 36 39 *Eddy current evaluation: (residualmagnetism at 1 msec) − (residual magnetism at 5 msec)

Embodiment 2

FIG. 12 is a graph of the results of measuring the effect that thesurface area ratio of the portion where the annular protrusion is incontact with the pole piece main component disposed directly therebelowhas on the magnetic field uniformity in the air gap of an MRI magneticfield generator (in which a permanent magnet structure comprisingR-Fe-B-based magnets is used as the field generation source) (thestructure in FIG. 1 was employed for the pole pieces). Specifically, thehorizontal axis is the distance in the radial direction from the centerof the air gap of the MRI magnetic field generator, and the verticalaxis is the magnetic field uniformity in this air gap.

The curves in the graph, from the top, indicate when the above-mentionedsurface area ratio was 100%, 80%, 70%, and 60%. Specifically, it can beseen that there is a pronounced drop in magnetic field uniformity as thesurface area of the portion where the pole piece main component touchesthe annular protrusion becomes smaller.

It is possible for the above-mentioned surface area ratio to be 80% orhigher in the present invention, and it was found that the effectdiscussed above for reducing residual magnetism and eddy current canstill be obtained without leading to a drop in magnetic fielduniformity.

Industrail Applicability

As is clear from the examples, because the MRI magnetic field generatorof the present invention makes use of pole pieces in which a maincomponent comprising a laminate of silicon steel sheets is effectivelycombined with an annular protrusion disposed on the side of the maincomponent facing the air gap, a static magnetic field having the desireduniformity can be formed in the air gap, and eddy current and residualmagnetism within the pole pieces, which are generated due to the effectof the pulse current flowing through the Gradient magnetic field coils,can be reduced without leading to a magnetically unsaturated state inthe vicinity of the annular protrusion.

What is claimed is:
 1. An MRI magnetic field generator comprising: ayoke; a pair of pole pieces disposed facing each other so as to form anair gap therein between; and a pair of permanent magnets supported bythe yoke, said permanent magnets for generating a magnetic field in theair gap, each permanent magnet having opposite ends, one end of eachpermanent magnate disposed facing the air gap and being directlyattached to the pole piece, and the opposite lens facing the yoke; thepole pieces being formed of a main component including a plurality oflaminated blocks, each comprising a plurality of laminated silicon steelsheets, wherein the laminated blocks of the silicon steel sheets arelaminated in a direction facing the pole pieces, and a magnetic annularprotrusion disposed on a side of the main component facing the air gap;wherein the magnetic annular protrusion has a side with a surface areaSa facing the main component and has a surface area Sb facing themagnetic annular protrusion and wherein the ratio of Sb/Sa is at least80% or higher.
 2. The MRI magnetic filed generator according to claim 1,further including a non-magnetic support member with high electricalresistance secured to on the side second proximate to the yoke forsupporting said main component.
 3. The MRI magnetic field generatoraccording to claim 2, wherein said non-magnetic support member comprisesa resin, bakelite, FRP, or another such non-metal.
 4. The MRI magneticfield generator according to claim 1, further comprising magneticannular support member divided into a plurality of sections in thecircumferential direction secured to a perpetual portion of the maincomponent.
 5. The MRI magnetic field generator according to claim 1,wherein said magnetic annular protrusion comprises laminated siliconsteel sheets.
 6. The MRI magnetic field generator according to claim 5,wherein said main component and magnetic annular protrusion comprisessilicon steel sheets laminated in the direction facing the pole pieces.